With rising environmental concerns, more attention than ever is being paid to the issue of surface casing vent flow in gas producing wells.
In a natural gas well, the subterranean gas is to be produced by flowing upward to the surface through the production casing of the well. Surface casing vent flow refers to movement of gas upward around the outside of the production casing and discharging into the atmosphere around the wellhead through an annular space between the production casing and the larger diameter surface casing that closes around the production casing from the ground surface downward over an upper portion of the production casing's height to protect shallow ground water formations. If all of the natural gas is being produced upward through a leak-free production casing, then the surface casing vent flow should be zero. To ensure that this vent flow of gas to the atmosphere remains acceptably low so as to minimize the environmental effects of the well, surface casing vent flow tests are performed to monitor the vent flow and compare the measured data against a maximum acceptable value, such as that set by a regulatory agency.
For example, one vent flow test involves shutting-in the annular space between the surface and production casings with a pressure relief valve set according to a maximum allowable shut-in pressure, for example as dictated by a regulatory agency, and using a pressure gauge or pressure recorder to monitor the pressure of the shut-in casing over a shut-in period. If within this period the pressure stabilizes at a value below the pressure relief valve setting, then the surface casing vent flow is considered acceptable. If the pressure relief valve setting is reached, then the vent flow is above the acceptable maximum, then the test is failed and taken as an indication of a problem that will need to be addressed through further action. Finally, if the pressure did not stabilize, the test is repeated with a longer shut shut-in period.
In order to be able to perform these tests, a gas well can be provided with a venting system. The annular space between the surface and production casing is closed off in a sealed manner near the ground surface and tapped at a location beneath this seal, where a hole or port in the surface sealingly communicates the annular space with a vent line that has its opposing end disposed above ground to exhaust vent flow gas above ground. A valve on the vent line is used to control the release of vent flow gas by opening and closing the line's passage from the well's annular space to the vent line exhaust outlet above ground. Normally the valve is left open so that vent flow gas is released into the atmosphere instead of building up pressure within the well, as sufficient pressure build-up could lead to failure of the surface casing and resulting contamination of groundwater. When a surface casing vent flow test is to be performed to measure the shut-in pressure, the valve is then closed.
In order to prepare a well for these shut-in tests, it is accordingly necessary to seal the annular space between the surface casing and the production casing sealed at the wellhead.
One method used for forming a seal between the casings is to measure an axial offset between the surface casing and the production casing extending downward therethrough and then produce an annular rubber seal using these measurements. A cut is then made through the ring shaped seal to form a split therein, so that the split ring can be placed around the production casing at a position below the wellhead valve equipment from which it is suspended in use. The two ends of this split-ring are then adhesively reconnected so that the ring is once again complete and now closes fully around the production casing. Two halves of a clamping device are positioned above and below the ring and bolted together therethrough, so that tightening of the bolts will flatten out the rubber ring expanding it radially to seal tightly against the outer surface of the production casing and the inner surface of the surface casing. However, a potential problem with this solution is that the resulting seal may not be able to handle significant pressures that may be experienced during shut-in testing for surface casing vent flow.
Accordingly, there is a desire for other methods of forming a seal between the surface and production casing of a gas well, and such methods may also prove useful in other applications also requiring sealing of annular space between an outer casing or tube and a inner tube extending within the outer tube along an axial direction thereof.